Jet engines which are currently used in the aeronautical industry and are commonly called turbine engines operate using a so-called Joule-Brayton thermodynamic cycle by constant-pressure combustion of a mixture of compressed air and fuel. The gases issuing from this combustion are then discharged at high velocity into the atmosphere through a nozzle, thus producing a propulsive force.
A considerable amount of R&D effort has been directed towards these turbine engines for the last sixty years and they have reached a mature stage of development. There is today little potential for improving them. Now, faced with environmental pressures and the rising price of fuel, the aeronautical industry must find new technological solutions to improve the performance of jet engines.
The use of jet engines which operate using a so-called Humphrey thermodynamic cycle seems to be a promising way ahead. In contrast with traditional turbine engines, the combustion takes place at a constant volume and no longer at a constant pressure. The theoretical potential of such jet engines has been known for a very long time but their practical application comes up against technological difficulties.
The best known constant-volume combustion jet engine is the wave rotor. The wave rotor operates according to the principle of a barrel. It consists of multiple enclosures arranged around the axis of a cylinder. The cylinder rotates between two fixed ends called end plates. Each of its ends comprises ports which control the flow of the gases, in particular to the compressor and the turbine. When the cylinder rotates, the enclosures are thus connected cyclically to the compressor and to the turbine.
In a first phase of the cycle, the enclosure is connected solely to the compressor. The enclosure then fills up with compressed gas and fuel.
This phase is followed by a phase during which the enclosure is closed, countering the circulation of the gases to the compressor or the turbine. Combustion is then performed in the enclosure. This combustion therefore takes place at a constant volume.
Lastly, the enclosure is connected to the turbine. The gases issuing from the combustion are then discharged towards the turbine.
In this type of jet engine, there is a considerable leakage of gases between the fixed ends and the cylinder, which considerably reduces the performance of such systems.
In order to reduce these losses, constant-volume jet engines exist using the Humphrey cycle in which the volume of the combustion chamber is maintained constant by valves.
The document FR2829528 describes such a jet engine which comprises multiple combustion chambers which are closed periodically by butterfly valves. The valves partially reduce the leakage of gases but, owing to the alternating closing and opening cycles, they are subjected to repeated impacts which, under the conditions of high temperatures in the order of 2000° C., entail rapid wear.
More generally, in the current constant-volume combustion jet engines there is a high risk of wear on the surfaces subjected to fluctuations in pressure and temperature.
Moreover, in addition to the poorer performance caused by the leaks of gas, there is also a loss of efficiency due to the many areas of highly turbulent flow.